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Name: Nelly
Status: Student
Age: 16
Location: N/A
Country: N/A
Date: N/A 

Our stomach has an enzyme called pepsin, which digests proteins into polypeptides and our small intestine also has an enzyme called trypsin which digests protein into polypeptides too. Since our stomach has an enzyme which can digest protein, why does protein digestion still take place in small intestine?

The enzymes you a referring to are generally called 'proteases', proteins that degrade other proteins. They do not do this randomly, but use a specific amino acid sequence to cleave. And that iwhy we produce different proteases in our digestive tract: to chop up as much as protein, in as small fragments as possible. After all, not every protein contains sufficient target sequences for trypsin or pepsin to generate small enough fragments.

Trudy Wassenaar

Nelly - Pepsin only begins the digestion of proteins as it breaks specific bonds, adjacent to precise amino acids; it leaves many other bonds intact. The enzymes in the small intestine finish the job, breaking the rest of the bonds that link amino acids together to make proteins. Is that what you need?

Ellen Mayo

Because proteins are very long complicated molecules. Proteins have 3 levels of complexity. The most complex level is the 3 dimensional level. Pepsin helps to "unwind" the proteins and breaks the bonds between the amino acids in certain places. The food passes to the small intestine and other enzymes break the bonds between different amino acids than pepsin. Because proteins are such complicated molecules it takes a long time and more than one enzyme to completely break them down into amino acids.


All polypeptides are composed of amino acids which have various side chains that distinguish the 20 different amino acids from each other. Some of these side chains carry a positive charge; some, a negative charge; and some, no charge at all. To break down a peptide to usable components the digestive system must be designed to attack all the different charges. In the acidic environment of the stomach, the negative side chains are addressed by pepsin.

In the more alkaline environment of the intestine, the positive side chains are digested by trypsin. Each of these enzymes are similar to each other but through evolution have been selected for stomach and intestine because pepsin works better in an acidic environment and trypsin works better in alkaline environments.

Cherie Breffeilh

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